Fix hanging test suite by closing per-loop Tortoise contexts

pwo.async_test runs every test in a fresh event loop and TortoiseMixin
builds one TortoiseContext per loop. Dropping the previous context when
the loop changed orphaned its aiosqlite connections; their non-daemon
worker threads then blocked threading._shutdown forever, so the suite
printed OK but the interpreter never exited.

Add TortoiseMixin.aclose() and a tests.helpers.async_test wrapper that
closes the context in a finally on the test's own loop, and switch the
test modules over to it.

server/tests/test_websocket.py

@@ -7,12 +7,11 @@ import unittest
 from httpx import ASGITransport, AsyncClient
 from httpx_ws import WebSocketDisconnect, aconnect_ws
 from httpx_ws.transport import ASGIWebSocketTransport
-from pwo import async_test
 
 from tavolo.app import app, game_store
 from tavolo.game import engine
 from tavolo.game.state import Card, GameState, PlayerState
-from tests.helpers import make_user, oidc_user, ws_users
+from tests.helpers import async_test, make_user, oidc_user, ws_users
 
 PLAYERS = ("alice", "bob", "carol", "dave")